1. Field of the Invention
The present invention relates to a vertical, collector-type high-pressure feed water preheater, with a desuperheater casing.
2. Description of the Prior Art
In the case of vertical, collector-type high-pressure feed water preheaters, which are heated by means of interstage bleed steam, it is possible, if the steam enters at a high temperature in the region of the outlet feed water collector, to provide a desuperheater casing in which the inflowing steam gives off so much heat that it is completely condensed at the end, that is at the feed water inlet header. In this design, the desuperheater casing forms a jacket which encloses the last portion of the serpentine tubes forming the preheater, upstream of the points at which they open into the outlet feed water collector, in a manner such that the steam is constrained to flow over the preheater tubes in the longitudinal direction, as a result of which good heat transmission occurs in this portion of the preheater, and the steam is cooled by the desired amount. At the end of this portion of the preheater, the steam emerges at right-angles to the adjacent leg of the preheater tubes, so that it can then condense completely over the remainder of the area of the serpentine tubes which form the preheater.
In the case of the known designs of vertical high-pressure feed water preheaters, of this type, the means for supporting the preheater tubes present problems, for which purpose there are typically provided split supporting plates, straight strips with and without webs, corrugated strips and comb-plates. The use of these devices creates the disadvantage that it is necessary to adjust these supporting structures to fit the tubes, and to weld them to the jacket of the desuperheater casing. This type of manufacturing procedure is complicated, and there is a danger of the tubes jamming, in which event large reaction forces can occur as the result of thermal expansions. Since a supporting structure of this kind significantly restricts the flow cross-section, comparatively high steam velocities occur there, which could give rise to erosion effects if condensation has already started in the desuperheater. Moreover, comparatively high pressure losses are associated with supporting structures of this kind.